To know what helmet size you are, you must measure the circumference of your head. For this you can help yourself, for example, with a measuring tape or a thread. Once that value is obtained, you will find your corresponding size in the following table.
Most helmets have two shells (outer part) for the entire range of sizes. If there are more shells, you can wear one that is more suited to the circumference of your head without having to carry the weight and size of one that is excessively large for you.